In monolithic radio frequency (“RF”) circuits, losses associated with the substrate impedance have deleterious effects on performance. For example, in RF switches built on bulk substrate technology with junction isolations, parasitic substrate impedance can result in degraded linearity and voltage imbalance across large branches of stacked semiconductor devices. Other deleterious effects may include higher parasitic losses suffered by passive components (e.g., transmission lines, inductors, etc.) reflected in lower Quality Factor, and unwanted crosstalk between devices or circuit blocks through the substrate.
Some of these effects experienced by the monolithic RF circuits when utilizing bulk substrate technology can be partially mitigated by using high resistivity substrates. However, in a conventional monolithic RF circuit employing a high resistivity substrate, fixed positive charges in the dielectric layers on top of the high resistivity substrate, can induce an inversion layer in the high resistivity substrate. For example, due to the low background doping of the high resistivity substrate, the fixed positive charges can cause an accumulation of high mobility carriers of opposite polarity under the dielectric layer. This inversion layer of high mobility carriers can significantly decrease the effective resistivity of the high resistivity substrate. For example, if 1000 ohm-cm silicon is used to build a high resistivity substrate, the effective resistivity of the substrate is found to be only approximately one fifth of that value (i.e., 200 ohm-cm) based on transmission line measurements over the substrate.
Another concern associated with high resistivity substrates is that large spacing is required to avoid leakage between junctions at different potentials. That is, the large depletion regions that surround biased junctions (where one side is the low doped substrate) require very large spacings to electrically isolate these junctions. A deep dielectric filled trench in the high resistivity substrate can significantly reduce the spacing to allow smaller products. While one or more deep dielectric filled trenches can help form isolated segments or islands in the high resistivity substrate to reduce the impact of the inversion layer, these islands of inverted charges can form p-n junctions with the underlying high resistivity substrate, which function as voltage dependent diodes that can adversely impact the linearity of the RF signals routed through the metal layers over the high resistivity substrate. In addition, the capacitive coupling between one or more of the metal layers and the high resistivity substrate would also become voltage dependent, thus further degrading the linearity of the RF signals.
Thus, there is a need in the art for mitigating substrate parasitics in a high resistivity substrate and improving RF signal linearity of monolithic RF circuits integrated on the high resistivity substrate.